Process for carbonizing cellulose fiber or the products thereof

ABSTRACT

Cellulose fiber or the product thereof is treated with a strength increasing agent selected from the group consisting of (A) at least one compound selected from ammonium sulfite, ammonium bisulfite, ammonium bisulfate, or ammonium thiosulfate; (B) a mixture of at least one compound selected from ammonium sulfate, ammonium bisulfate, ammonium sulfite, ammonium bisulfite, ammonium thiosulfate, ammonium sulfate, or ammonium imidosulfonate with at least one organic nitrogen base; and (C) a mixture of an organic nitrogen base and an acid selected from sulfuric acid, sulfurous acid or sulfamic acid.

United States Patent Miyamichi May 9, 1972 [54] PROCESS FOR CARBONIZING 21 Appl. No.: 869,196

[30] Foreign Application Priority Data Nov. 6, 1968 Japan ..43/81 178 Sept. 5, 1969 Japan.... .....44/70437 Sept. 5, 1969 Japan.... .....44/70438 Sept. 19. 1969 Japan ..44/74483 [52] US Cl. ..1 17/46 CB, 23/2091, 23/2094, 8/1 16.2, 117/228 [51] Int. Cl. ..C01b 31/07 [58] Field of Search ..l17/46 CB, 46 CC, 228; 23/2091 F, 209.4, 209.5; 8/1 16.2, 116

[56] References Cited UNlTED STATES PATENTS 3,305,315 2/1967 Bacon et a]. ..23/209.4

\1 sh/Z1 TENS/LE STRENGTH 3,297,405 l/l967 Sperk et al 23/2094 3,333,926 8/1967 Moyer et a1... ....23/209.4 3,441,378 4/1969 Didchenko .23/209.4 3,461,082 8/1969 Otani et a1. .23/2094 3,479,150 1 H1969 Gutzeit .1 23/209 4 3,479,151 ll/l969 Gutzeit .117/46 CB 3,527,564 9/1970 Moore et a1. ..23/2()9.5 3,532,466 10/1970 Johnson et a1 ..23/209.4

Primary ExaminerWilliam D. Martin Assistant E.\amInerM. Sofocleous Atrorne \'Birch, Swindler, McKie & Beckett [57] ABSTRACT Cellulose fiber or the product thereofis treated with a strength increasing agent selected from the group consisting of (A) at least one compound selected from ammonium sulfite, ammonium bisulfite, ammonium bisulfate, or ammonium thiosulfate; (B) a mixture of at least one compound selected from ammonium sulfate, ammonium bisulfate, ammonium sulfite, ammonium bisulfite, ammonium thiosulfate, ammonium sulfate, or ammonium imidosulfonate with at least one organic nitrogen base; and (C) a mixture of an organic nitrogen base and an acid selected from sulfuric acid, sulfurous acid or sulfamic acid.

6 Claims, 6 Drawing Figures Mom/r B4770 0F BASE r0 AMMO/V/UM SALT PATENTEDHAY 91972 3.661.616

sum 1- OF 4 25cm w/mn 6 6 TENS/LE .S7PE/VGTH Kg HEAT TREATMENT TEMPERATURE ("6) TENS/LE STRENGTH (Kg/2.5 cm Mom) 4\ Mom? RAT/0 OF 8435 r0 AMVO/V/UM SALT PROCESS FOR CARBONIZING CELLULOSE FIBER OR THE PRODUCTS THEREOF The present invention relates to a process for producing excellent carbonized fiber or the product thereof by treating cellulose fiber or the product thereof with a specific strength increasing agent or a treating agent prepared by adding a flame resistance improving agent to said strength increasing agent and thereafter heat-treating the thus treated cellulose fiber or the product thereof.

More particularly, the present invention concerns a process for carbonizing cellulose fiber or the product thereof characterized by treating cellulose fiber or the product thereof with a strength increasing agent comprising a specific compound or mixture mentioned below or with a treating agent obtained by adding a flame resistance improving agent to said strength increasing agent, thereafter carbonizing said cellulose fiber or product thereof by the heat treatment at a temperature of 200 to 350C. in an oxidative atmosphere and subsequently at a temperature of up to about l,000C. in an inert atmosphere and further if necessary, carbonizing or graphitizing the thus heat-treated fiber or product thereof by the heat treatment at a temperature of about 1,000C. or higher in an inert atmosphere.

A process for producing carbon fiber or graphite filament using cellulose fiber as a starting material has been known for a long time. Thomas Edison presented in US. Pat. No. 223,898 (1880) a process for preparing carbon fiber by dissolving cotton or flax in a zinc chloride solution, extruding this solution in an alcohol coagulating bath to obtain a fiber and heat-treating the thus obtained fiber. Further, W.R. Whitrey showed in US. Pat. No. 916,905 (1909) a process for graphitizing carbon fiber by the heat treatment at a temperature of 2,300C. or higher.

However, those actually produced in these days possess such defects as weak mechanical properties and too large porosity ratio so that the loss due to oxidation was large. The reasons for such defects are the facts that, in the case of pyrolyzing cellulose, the deterioration suddenly occurs particularly in the temperature range of 200 to 260C. and the decomposition accompanying the gas generation suddenly occurs in a temperature range of 260 to 500C. Therefore, if the heating rate is high, the degree of deterioration becomes high and the mechanical properties are reduced. Thus those having passed through such a stage, even if heated at a further high temperature, do not afford high quality carbon fiber or graphite filament.

By such reasons, a process for preventing the deterioration by heating cellulose fiber at an extremely low heating rate has been proposed. British Pat. No. 1,025,499 (1964), for example, employs the heating within a temperature of from 150 to 540C. at a heating rate of 10 to 30C./8 to 30 hours. Thus, this process requires 3 to 50 days for elevating the temperature up to 540C. Moreover, Japanese Pat. Publication No. l3,l l3/6l employs the heating to 400C. at a heating rate of 10 to 50C./hr. and thereafter the heating to 900C. at a heating rate of lC./hr. or less.

The present invention is to provide a new process which improves the above-mentioned conventional processes in the production of carbon-like fiber, carbon fiber, graphite fiber or products thereoffrom cellulose fiber or products thereof.

The object of the present invention is to provide a new treating agent or strength increasing agent which prevents the deterioration of cellulose due to the pyrolysis in the heat treatment of cellulose fiber or products thereof.

Another object of the present invention is to provide a process for preparing highly reinforced flexible carbon-like fiber, carbon fiber, graphite fiber or products thereof by treating cellulose fiber or products thereof with the above-mentioned strength increasing agent in the heating treatment of cellulose fiber or products thereof.

Further another object of the present invention is to provide a process for highly increasing the strength of the heat-treated fiber or the products thereof in the heat treatment at a temperature of 200 to 500 C. at which the deterioration of cellulose is particularly remarkable and in the heat treatment at further higher temperatures.

Further another object of the present invention is to provide an industrially extremely advantageous process which enables the heat treatment of high heating rate in the heat treatment of cellulose fiber or the products thereof.

In order to attain the above-mentioned objects, the present invention requires the treatment with a specific compound or mixture which is called a strength increasing agent. The strength increasing agent herein includes the following compounds and mixtures:

l. Ammonium sulfite, ammonium bisulfite, ammonium bisulfate and ammonium thiosulfate.

2. A mixture of ammonium sulfate, ammonium bisulfate, ammonium sulfite, ammonium bisulfite, ammonium thiosulfate, ammonium sulfamate or ammonium imidosulfonate with the later-defined nitrogen-containing base.

3. A salt obtained from sulfuric acid, sulfurous acid or sulfamic acid and the later-defined nitrogen containing base.

With respect to the strength increasing agent of paragraphs (2) and (3), the nitrogen-containing base includes urea, urea derivatives, thiourea, thiourea derivatives and amines such a urea (CO[NH thiourea (CS[NH 21 guanidine (NI-I:C[NH dicyandiamide (NI-I C:NH NH CN), dicyandiamidine (NI-I C: NI-I NH CO NI-I triethylamine [C I-I N), triethanolamine ([CH CH OH] N),

pyridine aniline NH2) and the like.

With respect to the above-mentioned strength increasing agent, the ammonium salt of paragraph (I) may be a mixture of two or more members thereof, and the strength increasing agent of paragraph (2) may be a mixture of one member of ammonium salts and two or more members of nitrogen-com taining bases or a mixture of two or more members of ammonium salts and one or more members of nitrogen-containing bases. Further the strength increasing agent of paragraph (3) may be prepared by adding two or more members of nitrogen-containing bases to one member of acids or by adding one or more members of nitrogen-containing bases to two or more members of acids.

With respect to a system comprising one member of acids and one member of nitrogen-containing bases and the above mentioned other combination system of one or more members of acids and one or more members of nitrogen-containing bases herein, the acid and nitrogen-containing base need not exist individually in a equimolar or equivalent amount, and one components thereof may exists in excess.

Every above-mentioned strength increasing agent is generally used in a form of aqueous solution thereof in the treatment of cellulose fiber or the products thereof. Ac cordingly, the said strength increasing agent seemingly is impregnated in and adheres to cellulose fiber or products thereof in a form of a salt, a mixture of the salt and nitrogen-containing base or a mixture of the salt and acid.

Figures are described below.

FIG. 1 shows the relationship between the strength of heattreated cloth obtained and the temperature of heat treatment, in the heat treatment of viscose rayon cloth;

FIG. 2, the relationship between the strength of heat-treated cloth obtained and the adhesion percentage of ammonium bisulfate, ammonium sulfite and ammonium thiosulfate to the starting material cloth, in the heat treatment of viscose rayon cloth precedingly treated individually with ammonium bisulfate, ammonium sulfite and ammonium thiosulfate wherein a represents the case of ammonium sulfite, b the case of ammonium thiosulfate and c the case of ammonium bisulfate;

FIG. 3, the strength of heat-treated cloth obtained and the composition of ammonium salt-nitrogen-containing base system strength increasing agent in the heat treatment of viscose rayon cloth precedingly treated with the said strength increasing agent; wherein a represents the case of ammonium sulfite-urea system, b the case of ammonium sulfite-thiourea system, c the case of ammonium bisulfate-urea system and d the case of ammonium bisulfite-triethanol-amine system;

FIG. 4, the strength of heat-treated cloth obtained and the adhesion percentage of sulfuric acid-nitrogen-containing base system strength increasing agent, in the heat treatment of viscose rayon cloth precedingly treated with the said strength increasing agent wherein a represents the case of sulfuric acidurea system, b the case of sulfuric acid-guanidine system, c the case of sulfuric acid-thiourea system, d the case of sulfuric acid-triethanolamine system, e the case. of sulfuric acidethylenediamine system and f the case of sulfuric acid-dicyandiamidine system;

FIG. 5, the relationship between the strength of heat-treated cloth obtained and the composition of sulfuric acid-nitrogencontaining base system strength increasing agent in the heat treatment of viscose rayon cloth precedingly treated with the said strength increasing agent, wherein a represents the case of sulfuric acid-urea system, b the case of sulfuric acidtriethanolamine system and c the case of sulfuric acid-thiourea system; and

FIG. 6, the relationship between the strength of heat-treated cloth obtained and the composition of sulfamic acid-nitrogencontaining base system strength increasing agent, in the heat treatment of viscose rayon cloth precedingly treated with the said strength increasing agent, wherein a represents the case of sulfamic acid-triethanol amine system, b the case of sulfamic acid-urea system, c the case of sulfamic acid-urea (40 percent system and d the case of sulfamic acid-thiourea system.

As is known, cellulose fiber, on pyrolysis, initiates its decomposition accompanying the weight loss at a temperature of about 150 to 160C. and the strength of the fiber decreases. Particularly, in the case of pyrolyzing cellulose fiber for a long period of time such as 1 hour or more, this tendency is remarkable and the strength of fiber suddenly falls at a temperature of 200C. or higher. FIG. 1 is an example which shows this tendency, and shows the relationship between the obtained tensile strength of heat-treated cloth and the temperature of heat treatment when viscose rayon diagonal cloth is heated up to the specified temperatures at a heating rate of C./min. and heat-treated at each temperature for 1 hour respectively. This figure shows that the strength of viscose rayon diagonal cloth suddenly falls at a high heat treatment temperature and that an inflection point of the strength exists in the vicinity of 280C.

In contrast thereto, the heat treatment of cellulose fiber soaked in the above-mentioned strength increasing agent includes the temperature of remarkable strength reduction at a temperature of 160 to 180C, but suddenly recovers the strength at a temperature of higher than 180C. In addition, this heat treatment of such processed fiber affords flexible heat-treated fiber having high strength at a temperature of 280C. or higher at which the heat treatment of cellulose fiber not soaked in such a strength increasing agent simply gives extremely low strength, in other words, flexible heat-treated fiber with low strength reduction, and in this case the strength increasing action of strength increasing agent relates to the adhesion percentage of strength increasing agent to the starting material cellulose fiber and the mixing ratio of ammonium salt and nitrogen-containing base or acid and nitrogen-containing base of the above-mentioned strength increasing agent of the present invention in the case of the strength increasing agents of paragraphs (2) and 3). Thus, by the above-mentioned findings, the present invention has been completed.

In view that such strength increasing action of the strength increasing agent of the present invention cannot be obtained by such an ammonium salt as ammonium nitrate, ammonium chloride, ammonium acetate, ammonium oxalate, ammonium formate, ammonium phosphorates or the like, or by such an ammonium salt as aluminum ammonium sulfate, nickel ammonium sulfate or zinc ammonium sulfate which contains a metal atom in the molecule and that these ammonium salt, rather reduce the strength of heat-treated fiber, it is considered that the compounds which exerts the strength increasing action are only such specific compounds as those of the present invention.

Such strength increasing action of the strength increasing agent of the present invention, in general, increases together with the increase of the adhering amount thereof to cellulose fiber or the products thereof. Too much adhesion thereof, however, involves such secondary disadvantages as the starting material fiber or the products thereof being hardened too much resulting in the inconvenience in the handling thereof or as the soaking treatment becoming difficult, but the strength increasing effect is unaltered.

When ammonium salt-nitrogen-containing base system or acid-nitrogen-containing base system is used as a strength increasing agent, it is observed that the strength of heat-treated fiber or the products thereof increases in harmony with the increase of the adhering amount of the strength increasing agent to the starting material fiber or the products thereof but that, when the adhesion percentage of strength increasing agent exceeds a certain value, the strength of heat-treated fiber or the products thereof decreases slowly or suddenly. With respect to such a strength increasing agent, the strength increasing action thereof also depends on the mixing ratio of nitrogen-containing base to acid or ammonium salt, and it is recognized that, with respect to every strength increasing agent, the most suitable mixing ratio or the preferable mixing ratio exists.

The strength increasing agent of the present invention having the above-mentioned strength increasing action is subsequently described concretely.

Firstly, the strength increasing action of ammonium salt is described. FIG. 2, when viscose rayon diagonal cloth is soaked in each aqueous solution of ammonium bisulfate, ammonium sulfite and ammonium thiosulfate, and thereafter heat-treated in airat 250C. for 2 hours and further at 300C. for 1 hour, show the relationship between the adhesion percentage of each ammonium salt to the starting material diagonal cloth and the strength of heat-treated cloth. This figure, excluding the case of ammonium bisulfate, shows that the strength of heat-treated cloth increases in accordance with the increase of adhesion percentage of ammonium salt.

In the case of ammonium bisulfate, when the adhesion percentage is not higher than about 10 percent, the strength of heat-treated cloth increases, but when it is higher than about 10 percent, the strength of heat-treated cloth decreases. Therefore, in the case of using ammonium bisulfate, the soaking treatment should be carried out so as to give about 10 percent of adhesion percentage. It is considered that such behavior of ammonium bisulfate is due to its brittle action against cellulose the action of acidic I-I existing in the molecule of ammonium bisulfate. For example, viscose rayon cloth molders if it is soaked in an aqueous solution of ammonium bisulfate, dried and allowed to stand at room temperature for 1 day or more without any further processing. In the heat treatment of cellulose fiber or the products thereof treated with ammonium bisulfate, such brittle action may also be involved in a series of steps of soaking, drying and heat treatment, resulting in the reduction of the strength of starting material fiber or the products thereof. After all, the highly strong heat-treated fiber or the products thereof may not be obtained.

Since ammonium bisulfite also contains acidic 1-1 in its molecule, it shows the same behavior as ammonium bisulfate and has substantially the same strength increasing action as ammonium bisulfate.

Every ammonium salt mentioned above may be used in the form of mixture. In this case, since the strength increasing action of each ammonium salt differs from each other, the said strength increasing action depends on the mixing ratio of each ammonium salt. However, this mixing does not recognizably exert an especial geometrical effect.

The strength increasing action of a mixture system of ammonium salt and nitrogen-containing base, namely, a mixture system of ammonium sulfate, ammonium bisulfate, ammoniurn sulfite, ammonium bisulfite, ammonium thiosulfate, am-

monium sulfamate or ammonium imidosulfonate and each 5 nitrogen-containing base, is described below. Firstly, the relationship between the adhesion percentage of such a strength increasing agent to cellulose fiber or the product thereof and the obtained strength of heat-treated fiber or the products thereof is mentioned.

Table 1 shows the results when viscose rayon diagonal cloth (thickness 0.5 mm) is treated with strength increasing agents of ammonium imidosulfonate and ammonium imidosulfonateurea (weight ratio 2:1) system, and heat-treated in air at 250C. for 2 hours and further at 300C. for 2 hours. The

salt-nitrogen-containing base system or the later-mentioned acid-nitrogen-containing base system strength increasing agent, and may be a general phenomenon ofsuch cases.

Subsequently, with respect to the ammonium salt-nitrogencontaining base system strength increasing agent, the in- TABLE 1 Ammo- Adhesion 0 6.4 13.7 20.9 33.2 52.1) 70.2

nium

imido sullonate. Tensile strength. 2.7-3.1 6.0 81 12.1 12.) 1 19.1

Ammo- Adhesion 0 22.7 376 42.5 51.1 68 78.8

nium

imido- Sull'onate Tensile 2.7-3.1 12.5 15.7 17.2 14.3 7.0 3.2 urea strength.

1 Percentage.

Z Kg./2.5 em. ofwidth.

fluence of the mixing proportion thereof on the strength of heat-treated cloth is described.

Tables 2 and 3 show the results when viscose rayon diagonal cloth (thickness; 0.5 mm) is treated with a strength increasing agent prepared by mixing urea, thiourea or triethanol amine with ammonium sulfate, ammonium sulfamate or ammonium imidosulfonate, and thereafter heattreated in air at 250C. for 2 hours and further at 300C. for 2 hours.

TABLE 2 Treatment with strength increasing agent Heat-treated cloth Adhesion Tensile percentstrength age of Heating (longi- Elongation Total annno- Adhesion weight tudinal) (longiadhesion nium imipereentloss (kg/2.5 tudinal) Mixing ratio of base to ammonium (percentdosulage of (percentcm. of (percentsalt (weight ratio) age) fonate base age) width) ago) Non-treated a c 75 7h 2. 7-3. 1 5. 2-5. 1) Ammonium iinidosulfonate/urea:

100: 25. 8 25. 5 0 63 fl. 7 11. J 32.7 20.2 0.5 51 11.2 11.6 37. (i 225. 12 5 52 15.7 12. t) 50. El 2!). 1 .21. 8 12. 1 10. 4 (14.1] 32. U 32 0 55 10. 8 11). 0

28. 7 28. 7 U 63 11. 6 11. 2 35. 3 28. .2 7.1 53 11. 8 10. 0 42. 5 28. 3 14. 2 50 15. 0 J. U 5.2. 2 2) 8 4 4.) 11. 5 8. .2 68.1) 34 4 34. 5 46 10, 11.0 Ammonium imidosnlionate/triethanol amine:

.24. 0 19. 2 4. 8 61 12 7 l1. 4 31.6 21.1 10.5 58 3 8 5.5 42. 1 24. 0 18. 1 57 4 7 5. H 50. E) 25. 5 25. 4 57 2 o 1. 2

TABLE 3 Treatment with strength increasing agent Heattreated cloth Adhesion strength percent- Heating (longi- Elongation Total age of Adhesion weight. tudinal) (longiadhesion ammopercent less kg./2.5 tudinal) Mixing ratio of base to ammonium (percentnium age 01 (percentcm. of (percent salt (weight ratio) age) salt base age) width) age) Non-treated -76 2. 7-3, 1 5. 25. 1' Ammonium sulfate/urea:

:0 o 24.4 24.4 0 63 10.5 11.11 100225" 31. 3 25.1 15. 2 5'. 13.1 16. .2 100:50 34. 9 23.3 11. 6 5.) 14. 8 16. O 100:75 45.7 26.1 10.6 58 13.4 15.8 100:100 51,0 25.5 25.5 57 12.-l 15.6 Ammonium sulfamate/nrea:

Such a phenomenon as the proportion of strength increment of mixture system being higher than the case of single ammonium salt system and as the strength decreasing at a high The results in Tables 2 and 3 show that, in the case of using singly ammonium sulfate, ammonium sufamate or ammonium imidosulfamate, the strength of heat-treated cloth increases by adhesion percentage is also recognized in other ammonium 75 about three times as much as the strength of heat-treated cloth obtained from the non-treated starting materialcloth. In the case of adding urea, thiourea or triethanolamine to the abovementioned ammonium salts, the strength increasing action in every case is larger than that of the corresponding case of single ammonium salt employment. At a suitable mixing ratio, the strength of heat-treated cloth increases by 4 times or more as much as that of the case without such a treatment, and decreases via the maximum value when the'mixing ratio is changed.

FIG. 3 shows the case of mixing urea, thiourea or triethanolamine with ammonium sulfite and mixing urea with ammonium bisulfate. FIG. 3, when viscose rayon diagonal cloth is soaked in each aqueous solution of the above-mentioned strength increasing agents so as to produce almost constant adhesion percentage of ammonium salt and to vary the adhesion ratio of base, wrung, dried, and thereafter heattreated in air at 250C. for 2 hours and further at 300C. for 1 hour, shows the relationship between the strength of heattreated cloth and the composition of each strength increasing agent. Provided that, in the said heat-treatment, the adhesion percentages of ammonium salt to the starting material diagonal cloth, with respect to each strength increasing agent, are as follows:

Ammonium sulfite-urea system adhesion percentage of ammonium sulfite 18.0% Ammonium sulfite-thiourea system adhesion percentage of ammonium sulfite 17.5% Ammonium sulfite-triethanolamine system:

adhesion percentage ofammonium sulfite 16.0% Ammonium bisulfate-urea system adhesion percentage of ammonium bisulfate 23.5%

creases, the strength increasing action thereof is strengthened.

When the molar ratio of base to ammonium salt is from 0.75 to 1.00, a peak of strength increasing action is obtained, and the strength increasing agent of this molar ratio gives high strength to heat-treated cloth by 4 to 7 kg/2.5 cm of width higher than the case of using singly ammonium salt. With respect to ammonium sulfite-triethanolamine system, when the mixing ratio of triethanolamine is 1.00 or higher by molar ratio, the effect of base which promotes the strength increasing action of ammonium sulfite appears.

It is noteworthy that, as is clear from FIG. 2, the strength increasing action of ammonium bisulfate, which is weaker than that of ammonium sulfite, ammonium thiosulfate or the like is extremely strengthened by the addition of base to said ammonium bisulfate so as to give the strength increasing action which is comparable to the strength increasing action of ammonium sulfite, ammonium thiosulfate or the combinations of these ammonium salts with a base. This is because that acidic H in an ammonium bisulfate molecule, which seemingly exerts deterioration or brittle action to cellulose fiber materials, is neutralized by such a base as urea or the like to form the salt thereof, therefore, the thus produced salt behaves in the same way as such a neutral salt as ammonium sulfite or the like, and, in addition, the effect of base obtained when such a base as urea is added to such a neutral salt as ammonium sulfite, ammonium thiosulfate or the like is exhibited.

With respect to these strength increasing agents of mixture system, in the case of using ammonium thiosulfate as the ammonium salt component, the effect of base almost identical with that of the cases of other ammonium salt, can be obtained as well. The case of ammonium thiosulfate-base system is to be described in Examples below. I

FIG. 3 shows the results of the case where the adhesion percentage of ammonium sulfite is from 16 to 18 percent, while, as is clear from FIG. 2, the strength of heat-treated cloth of the case of single ammonium sulfite system increases together with the increase of the adhesion percentage of said ammonium sulfite to the starting material cloth. Thus, in this experiment of FIG. 3, the treatment of further increasing the adhesion percentage of ammonium sulfite may afford the further higher strength to the heat-treated cloth. However, as is deducible from the results in Table I, if the adhesion percentage is too high, the strength of heat-treated cloth rather decreases, and thus the adhesion percentage exceeding the limit should be avoided. The most suitable adhesion percentage of such a mixture system may easily be determined by experiment. Other ammonium salt-base systems may be under the identical conditions, and the most suitable adhesion percentage with respect to each system may easily be determined by experiment.

With respect to the above-mentioned ammonium salt-base system strength increasing agents, the most effective base, when combined with ammonium salt, is urea and thiourea. Triethanolamine, guanidine and triethylamine, in this case, belong to a second class as the compound exerting the base effect. Dicyandiamide, dicyandiamidine, aniline, pyridine and the like belong to the group of compounds exerting relatively low base effect.

The above-mentioned ammonium salt-nitrogen-containing base system strength increasing agent is not limited within the mixture systems comprising only one member of ammonium salts and only one member of nitrogen-containing bases, but includes the systems obtained by mixing two or more members of nitrogen-containing bases with one member of ammonium salts and the systems obtained by mixing one or more members of nitrogen-containing bases with two or more members of ammonium salts.

Subsequently, the strength increasing action of the systems comprising sulfuric acid, sulfurous acid or sulfamic acid and a nitrogen-containing base is described.

FIG. 4, when viscose rayon diagonal cloth is soaked in a respective aqueous solution of sulfuric acid-urea system (molar ratio; 1:2.75), sulfuric acid-thiourea system (molar ratio; 1:1), sulfuric acid-triethanolamine system (molar ratio; 1:3), sulfuric acid guanidine [NI-I:C(NH -H SO,), sulfuric acid ethylenediamine [NH CH CH NH -H SO,) and sulfuric acid-dicyandiamidine [NI-I CS NH NH CONH :I-I SO -2I-I O), wrung, dried and thereafter heat-treated in air at 250C. for 2 hours and further at 300C. for 1 hour, shows the relationship between the adhesion percentage of each strength increasing agent and the strength of heat-treated cloth. FIG. 5, when viscose rayon diagonal cloth is soaked in each aqueous solution of sulfuric acid-urea system, sulfuric acid-thiourea system and sulfuric acid-triethanolamine system strength increasing agents so that the adhesion percentages of sulfuric acid which is the acid component of the said strength increasing agents, are almost constant such as 15.0 percent, 1 1.3 percent and 5.9 percent, respectively, and so that the adhesion percentages of base components thereof are varied, and thereafter heat-treated in the manner as above, shows the relationship between the mixing ratio of each strength increasing agent and the strength of heat-treated cloth.

FIG. 6 shows the case of sulfamic acid-base system. FIG. 6, when viscose rayon diagonal cloth is soaked in an aqueous solution of respective strength increasing agent prepared by mixing urea, thiourea or triethanolamine with sulfamic acid at respective suitable mixing ratio and subsequently heat-treated in air at 250C. for 2 hours and further at 300C. for 1 hour, shows the relationship between the strength of heat-treated cloth and the composition of the above-mentioned strength increasing agent, wherein the adhesion percentage of the strength increasing agent of respective mixing ratio to the starting material diagonal cloth is controlled so as to make the adhesion percentage of sulfamic acid almost constant (about 20 percent) and the adhesion percentage of base varied. With nun:

respect to sulfamic acid-urea system, the case where the adhesion percentage of sulfamic acid is about 40 percent is also shown. In the figure, C represents this case.

The above-mentioned FIGS. 4 to 6 show the fact that, in the case of such strength increasing agents, the strength of heattreated cloth also relates to the adhesion percentage of strength increasing agent and to the composition thereof, as in the case of the aforesaid single ammonium salt system or ammonium salt-nitrogen-containing base system.

That is, with respect to the relationship between the adhesion percentage and the strength, FIG. 4 shows that the strength of heat-treated cloth increases in harmony with the increase of adhesion percentage of strength increasing agent and that, when the adhesion percentage exceeds a certain value, the strength gradually or suddenly decreases. In the case of sulfamic acid-urea system, FIG. 6 shows that, when the mixing molar ratio of urea to sulfamic acid is about 0.5 to 1.5, preferably about 0.8 to L5, sulfamic acid with about 40 percent adhesion percentage exerts stronger strength increasing action than that with about percent adhesion percentage.

With respect to the relationship between the composition of strength increasing agent and the strength, FIGS. 5 and 6 show that, with respect to every strength increasing agent, the most suitable or preferable mixing ratio exists. This most suitable or preferable mixing ratio is not necessarily limited within the ratio obtained when acid and base exist in equimolar amount, but includes the case where the acid or base exists in excess by molar ratio.

The strength increasing agent of sulfurous acid-base system behaves in the same way as that of sulfuric acid-base system and exerts almost identical strength increasing action. This is described in Examples below.

In FIG. 6, when base is not added at all, but when sulfamic acid is singly used, the strength of heat-treated cloth is small. This fact is seemingly because that sulfamic acid may deteriorate the starting material cloth during the sulfamic acid soaking step or a series of steps after the said soaking step.

The above-mentioned acid-nitrogen-containing base system strength increasing agent is not limited within the system comprising only one member of acids and only one member of nitrogen-containing bases, but includes the systems obtained by adding 2 or more member of nitrogen-containing bases to one member of acids and the systems obtained by adding one or more members of nitrogen-containing bases to two or more members of acids.

With respect to the above-mentioned acid-nitrogen-containing base system strength increasing agents, if urea, thiourea, guanidine, triethanolamine or the like as a nitrogen-containing base is combined with an acid, extremely or relatively strong strength increasing action can be exerted. In contrast thereto, dicyandiamide, dicyandiamidine, triethylamine, aniline, pyridine or the like belongs to a compound which exerts relatively weak strength increasing action.

With respect to the ammonium salt-nitrogen-containing base system strength increasing agents and acid-nitrogen-com taining base system strength increasing agents described in detail above, a nitrogen-containing base in itself, which is combined with acid or ammonium salt, does not possess strength increasing action. Further, even when the nitrogencontaining base is combined with acid or ammonium salt, if the said acid component or ammonium salt component is hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, trifluoroacetic acid, ammonium nitrate, ammonium chloride, ammonium acetate, ammonium oxalate, ammonium formate or the like, the strength increasing action is not also exerted. Therefore, the above-defined acids or ammonium salts which exert strength increasing action by the combination thereof with nitrogen-containing base are only such specific compounds as the acids or ammonium salts of the present invention.

If cellulose fiber or the product thereof is treated with a strength increasing agent which exhibits such an effect as mentioned above, even if heat-treated at any temperature, the

higher strength is obtained than the heat-treated article of cellulose fiber or the product thereof which has not been treated with such a strength increasing agent. Such an effect is to be clarified in Examples described below.

The present invention relates to a process for carbonizing the cellulose fiber or the products thereof preceedingly treated with a strength increasing agent described in detail above by the heat treatment at a temperature of 200 to 350C. in an oxidative atmosphere and subsequently at a temperature up to about 1,000C. in an inert atmosphere and, if necessary, carbonizing or graphitizing the thus obtained carbonized fiber or the products thereof by the further heat treatment at a temperature not less than about 1,000C. in an inert atmosphere. The above-mentioned treatment of starting material fiber or the products thereof with a strength increasing agent in advance to the heat treatment thereof enables the heat treatment at such a high heating rate as the heating up to a temperature of about l,O00C.( in an inert atmosphere) after the heat treatment at a temperature of 200 to 350C. in an oxidative atmosphere being 1 to 5C/min, which has not been achieved in any way by the conventional processes. Thus, this is an industrially extremely advantageous process. In view the fact that the treated substance with a strength increasing agent of the present invention, even when heated at any heating rate, always exerts higher strength and results in higher yield than the non-treated substance when the two are compared at the same heating rate, it is self-evident that the heating rate of the said treated substance is not necessarily limited within the above-mentioned treating rate. In general, since heat treat ment at a lower heating rate results in heat-treated fiber or the products thereof having higher strength, it is natural that not only this invention is restricted to said heating rate but also this invention includes the case of heat treatment at a heating rate of less than lC/min.

Carbon fiber or graphite fiber with a carbon content of about 95 percent or above can be produced by heat-treating the carbon-like fiber prepared according to the above-mentioned process of the present invention (carbon content: up to about percent) at a temperature of higher than about 1,000C. according to the conventional process. Even in this case, however, if the highly strong carbon-like fiber produced by the process of the present invention is used as the starting material, the stronger carbon fiber or graphite fiber than the conventional carbon or graphite fiber can evidently be obtained.

The present invention, as is described above, affords a process for heat-treating the cellulose fiber or the products thereof treated with a strength increasing agent, in at first an oxidative atmosphere and subsequently at a higher temperature in an inert atmosphere, and the oxidative atmosphere herein is generally represented by air. As an inert gas, such known inert gases as nitrogen, helium, argon, carbon dioxide and the like may be used.

In case the present invention is applied for the industrial large-scale heat-treatment, air incorporated in the heat treatment furnace, air initially contained in the cloth or oxygen contained in an inert gas may cause the deterioration of heattreated fiber or the products thereof due to the oxidation by said air or oxygen. Such a heat treatment may often afford the lower strength to the heat-treated fiber or the products thereof than the experimental heat treatment where the heat treatment atmosphere is completely replaced by an inert gas. In order to avoid this disadvantage, the starting material fiber or the products thereof may be soak-treated with a strength increasing agent, to which is precedingly added such a com pound as ammonium phosphates, guanidine phosphate, aluminum ammonium sulfate, tetrakis (hydroxymethyl) phosphonium chloride (THPC) and the like, which is called as a flame resistance improving agent clarified in the present inventors invention relating to the process for the production of flame-resistant fiber.

That is, as is clear from Table 4, the soaking treatment of the starting material cellulose fiber or the products thereof with an aqueous solution of the above-mentioned compound called as a flame resistance improving agent, even if the thus soaked fiber or the products thereof are heated-treated in air, affords the heat-treated fiber or the products thereof which do not involve not only the combustion but also the ember combustion and the reduction to ashes, in other words. the heattreated fiber or the'products thereof excellent in oxidation resistance. Accordingly, if the present invention is applied after treating cellulose fiber or the products thereof with a treating agent prepared by adding the above-mentioned flame resistance improving agent to a strength increasing agent of the present invention having been described in detail, even if oxygen is contained in an inert atmosphere, the heat-treated fiber or the products thereof excellent in oxidation resistance due to the presence of flame resistance improving agent can be produced, and thereby the oxidative deterioration due to said oxygen or air contained in the starting material fiber or the products thereof may be inhibited so as to give preferable results.

Table 4 shows the results of the cases where diammonium .hydrogen phosphate, triammonium phosphate, guanidine phosphate, THPC and aluminum ammonium sulfate are employed as a flame resistance improving agent. The case of ammonium dihydrogen phosphate also exerts the identical action. In this case, 2 kinds or more of flame resistance improving agents may simultaneously be used, and the amount of addition thereof may be small as compared with that of strength increasing agent.

TABLE 4 Heat-treated cloth Flame resistance improving agent Tensile strength Weight (longitudinal) Flame Adhesion loss (kg/2.5 cm. resist- Kmrl percentage (percent) olwidth) ance Nrmr: 78 5.8 C 8 6!) 4.3 A lu'ummoniumhy- 16 52 3.5 A tlrogcn phosphate. 24 48 2.2 A 32 38 1.5 A 8 5!) 2.1 A 'Iriammonium l6 53 2.3 A phosphate 24 46 2.9 A 32 40 2.5 A Guanidine 8 68 3.1 A phosphate .i 16 3.2 A v 8 2.6 A Tmc --t 16 51 3.8 A Aluminum nnnnoi 8 68 3.1 A nium sulfate ..l 16 62 3.2 A Note: Starting material cloth, viscose rayon diagonal cloth. Conditions of heat treatment: 250 C., 1 hour plus 300 C., 2 hours (in air). Flnnn resistance: A. not burning into a flame, no ember combustion, no reduction to nsln s; B, not burning into n flame, no ember combustion, {vdufing to ashes; 0, not burning into a flame, ember-burning, reducing it) HS 1(5.

The condition of heat treatment of the present invention should suitably be determined depending on the texture, shape, etc. of the starting material fiber or the products thereof. For example, when a big rattan, thick woven stuff, thick nonwoven cloth or felting is heat-treated, heat generated by the pyrolysis thereof is liable to be stored up inside the texture thereof, and thereby the abnormal heat generation is involved so that the temperature controlling may sometimes become difficult. in this case, such a method as reducing the heating rate should be applied.

The properties concerning the strength of heat-treated fiber or the products thereof obtained by the present invention, as mentioned above, depend on the kind of strength increasing agents, the adhesion percentage thereof, mixing ratio thereof, heat treatment conditions and so on, and in addition thereto relate to the micro-structure of starting material cellulose. That is, generally, the higher the degree of orientation of starting material cellulose, the larger the strength of the heat- 0 products thereof are not limited within the fiber or the treated fiber or the products thereof and the smaller the elongation thereof. The heat-treated fiber of such a highly crystalline cellulose fiber as polynosic fiber, cotton or the like is generally brittle, but such brittleness can be avoided if such crystalline starting material fiber is precedingly treated according to such a known method as mercerizetion or the like to reduce the degree of crystallization thereof and then heattreated.

Of the present invention, the starting material fiber or the products thereof singly comprising cellulose fiber, but include the fiber or the products thereof which is prepared by mixing such a known fiber capable of being carbonated by pyrolysis as polyacrylonitrile fiber, polyvinylalcohol fiber or the like with cellulose fiber. Of the present invention, cellulose fiber includes not only the fiber singly comprising cellulose fiber but also all of the fibers composed of the every above-mentioned materials, and represents every kind of the above-mentioned fibers.

As is described in detail above, the present invention, in the production of carbon-like fiber, carbon fiber, graphite fiber or the products thereof from cellulose fiber, enables the preparation of carbon-like fiber or the products thereof having the strength several times higher than the case of using nontreated cellulose fiber as a starting material even at a temperature of 200 to 500C. which belongs to the most dangerous temperature range wherein the mechanical properties of the carbonated fiber may be deteriorated, by precedingly treating cellulose fiber or the products thereof with a strength increasing agent provided for in the present invention or a treating agent prepared by adding a flame resistance improving agent to said strength increasing agent, and subsequently by heattreating the thus treated fiber or the products thereof at a temperature of 200 to 350C. in an oxidative atmosphere and thereafter at a temperature of up to about 1,000C. in an inert atmosphere, and thereby, enables the highly efficient production of carbon fiber, graphite fiber or the products thereof having the further higher strength than that of the conventional art, by the heat treatment of carbonation or graphitization at a further higher temperature.

The present invention is further described below in accordance with examples, but the present invention naturally is not limited by these examples.

EXAMPLE 1 Diagonal cloth comprising viscose rayon fiber (denier of monofilament: 1.5 d) spun yarn (thread density: longitudinal, 36 threads/2.5 cm.of width, transverse, 36 threads/ 2.5 cm. of width; weight 280 g/m) was soaked in an aqueous solution dissolving ammonium sulfite and ammonium dihydrogen phosphate (30 weight percent based on the weight of ammonium sulfite), and the thus soaked cloth was wrung at a wringing percentage of l00 percent and subsequently dried at C. for 3 hours so as to obtain a treated cloth having an adhesion percentage of solid component being 57.6 percent. The thus treated cloth was heated-treated in air in a hot wind drying oven at 250C. for 2 hours and further at 300C. for 1 hour, and thereafter the thus heat-treated cloth was washed and dried. The thus obtained heat-treated cloth in air had a weight loss of 52.1 percent, a tensile strength of 20.2 kg/2.5 cm. of width and the above-defined flame resistance being A, and was the heat-treated cloth excellent in durability to oxidation Subsequently, the said heat-treated cloth in air was charged in a stainless steel cylinder equipping a nitrogen charging inlet and an opening connected with a vacuum pump, and the air in the cylinder was replaced by nitrogen by the several times repeated evacuation and nitrogen introduction. Thereafter the cloth in the cylinder was heated at a heating rate of 5C/min. up to 1,000C, and heat-treated at the said temperature for 1 hour. The thus obtained heat-treated cloth had a weight loss of 52.7 percent (based on the weight of the above-mentioned heat-treated cloth in air) and a tensile strength of 6.66 kg/2.5 cm. of width.

In contrast thereto, the non-treated said starting material diagonal cloth was also heat-treated in air in the same way as above (weight loss 72.0 percent tensile strength: 6.4 kg/2.5 cm. of width), and thereafter the thus heat-treated cloth was heated in nitrogen up to 1,000C. and heat-treated at this temperature for 1 hour. The thus obtained heat-treated cloth had a weight loss of 53.6 percent (based on the weight of the heattreated cloth in air) and a tensile strength of 1.8 kg/2.5 cm. of width.

EXAMPLE 12 The same starting material diagonal cloth as in Example 1, was soaked in an aqueous solution of treating agent comprising 1,000 g. of ammonium sulfate, 500 g. of urea, 75 g. of tetrakis (hydroxymethyl) phosphonium chloride and 3,500 c.c. of water, wrung by a mangle and then dried so as to obtain treated cloth with the adhesion percentage of solid component being 50.4 percent. Subsequently, the thus treated cloth was heat-treated in air in the same way as in Example 1 so as to obtain heat-treated cloth in air having a weight loss of 53.7 per cent, a tensile strength of 18.8 kg/2.5 cm. of width and the flame resistance being A. Thereafter, the said heat-treated cloth in air, in the same way in nitrogen as in Example 1, was heated at a heating rate of C/min. up to 1,000C. and heattreated at this temperature for 1 hour. The thus obtained heattreated cloth had a weight loss of 52.1 percent (based on the weight of the above-mentioned heat-treated cloth in air) and a tensile strength of 9.84 kg/2.5 cm. ofwidth.

EXAMPLE 3 The same starting material cloth as in Example 1 was soaked in an aqueous solution of treating agent comprising 410 g. of sulfurous acid (net weight in sulfurous acid water), 750 g. of urea, 30 g. of diammonium hydrogen phosphate and 2,000 cc. of water, wrung by a mangle and then dried so as to obtain the heat-treated cloth with the adhesion percentage of solid component being 34.3 percent. Subsequently, this at a heating rate of 5C/min. up to l,000C. and heat-treated at this temperature for 1 hour. The results are shown in the following table.

Heat-treated cloth at 1,000 f. Strength increasing agent Tensile Adhesion weightless strength. pcrccntperccntkg./2.5 t'm. Kind age ago of width Ammonium sulfate-aniline (weight ratio; 2: 1) 41. 1 7. 13 5. (J5 Ammonium sulfamatc-urea (weight ratio: 4:1) 40.3 73.1 7. 71 Ammonium imidosulfonatethio urea (weight ratio; 2: 1) 50. 0 74. 3 11.03 Ammonium thiosulfatc-urca (molar ratio; 1:1) 37. 4 43. 7 7. 4 Ammonium sullitotricthanolamino (molar ratio; 1:1.25) 52.11 75. 5 3. Ammonium hisnlfntc-urca (molar ratio; 1:1) 35. 0 74.1 01

EXAMPLE 5 The same starting material diagonal cloth as in Example 1 was soaked in an aqueous solution of each strength increasing agent, taken out, wrung and dried. Thereafter, the thus treated cloth was heat-treated in a hot wind drying oven under the same conditions as in Example 1, and then the thus heat treated cloth was washed and dried so as to obtain the heattreated cloth in air having the flame resistance of B. Subsequently, this heat-treated cloth in air was charged in the same stainless steel cylinder as that of Example 1, the air in which was then replaced by nitrogen, thereafter heated at a heating rate of 5C/min. up to 1,000C. and heat-treated at this temperature for 1 hour. The results are shown in the following table. Provided that the weight loss of heat-treated cloth at 1,000C. is a value based on the weight of heat-treated cloth in air.

llcat-trcatcd cloth Heat-treated cloth Strength increasing agont in air at 1,000" (1.

'lonsilo 'lonsilo strength Weight strength Adhesion Weight kg./2.5 cm. loss kg./2.5 cm. Kind percentage loss of width (percent) of width Ammonium sulfito 40. 3 54. 5 11.3 53. 3 15. 53 Ammonium thiosullato 66. 6 35. 8 1G. 3 58. '2 1'. 00 Ammonium bisulfatc 11. 5 61.7 11. 2 57. 4 3. 60 Sulfuric acid-urea (molar ratio 122.75) 39. 6 44. 7 25. 4 53. U s. '20 Sulfuric acid-thiourea (molar ratio 1:2) 34.3 48. 8 10. 6 53. 5 4. 58 Sulfuric acid triethanolamino (molar ratio 1:3) 33.4 56. 2 15.0 57. S 4. 61 Sulfuric acid-aniline (molar ratio 1:2) 25. 1 60. 6 8.4 56.0 2. 56 Sulfuric acid.guanidinc 24. 7 52. 1 16. 0 52.11 6. 23 Sulfuric acidethylenediaminc" 16.0 59. 4 10.6 52. 3 3. '28 Sulfuric acid dicyandiamidine t 13. 9 61. 5 8.7 57. 7 '1. I14 Sulfamic acid-aniline (molar ratio 1:1) 53. 4 37. 3 13. 9 53. 8 5. 23 Sulfamic acid-tn'ethamol amine (molar ratio 1:1. 38. 7 46. 8 16. 0 55. 3 5. 83 Sulfamic acid-dicyandiamidc (molar ratio 1:1) 32. 4 45. 1 13. 4 53. 5 4. T2

EXAMPLE 6 treated cloth was heat-treated in air in the same way as in Example 1 so as to obtain the heat-treated cloth in air having a weight loss of 44.6 percent, a tensile strength of 18.8 kg/2.5 cm. of width and the flame resistance of A. Thereafter, the said heat-treated cloth in air, in the same way in nitrogen as in Example 1, was heated up to l,0O0C. and heat-treated at this temperature for 1 hour. The thus obtained heat-treated cloth had a weight loss of 51.7 percent (based on the weight of the above-mentioned heat-treated cloth in air) and a tensile strength of 9.73 kg/2.5 cm. ofwidth.

EXAMPLE 4 The same starting material diagonal cloth as in Example 1 was soaked in each aqueous solution of strength increasing agents, taken out, wrung and dried. Thereafter the thus treated cloth was heat-treated in a hot wind drying oven under the same conditions as in Example 1, and then the thus heattreated cloth was washed and dried so as to obtain the heattreated cloth in air having the flame resistance of B. Subsequently, this heat-treated cloth in air was charged in the same stainless steel cylinder as that of Example 1, the air therein was then replaced by nitrogen, and thereafter heated thus treated tow was heat-treated in air at 220C. for 1 hour, at

250C. for 2 hours and further at 300C. for 1 hour so as to obtain heat-treated tow in air having a heating weight loss of 41 percent. Meanwhile, the above-mentioned starting material tow which has not been soak-treated with the above-mentioned treating agent was also heat-treated in the same way as above so as to obtain the heat-treated tow in air with a heating weight loss of 68 percent.

Subsequently, the thus obtained 2 kinds of heat-treated tow in air were individually charged in a stainless steel cylinder, the air in which were replaced by nitrogen. The said cylinders were placed in an electric furnace, then heated individually at such a kinds of heating rates as (A) 8C/min. at temperatures of below 500C. and 2C/min. at temperatures of above 500C. and (B) 8C/min. at temperatures of below 500C. and C/min. at temperatures of above 500C. up to 400C., 600C, 800C. and 1,000C., and heat-treated at each temperature for 1 hour. Thereafter, while introducing nitrogen,

the cylinder was cooled, and when the temperature thereof 5 passing through the zone at 220C. and the zone at 260C. in a continuous heating furnace equipping infrared heaters so that the total passing time is 1.5 hours. Subsequently, this bundle of heat-treated fiber in air was charged in the same stainless steel cylinder as in Example 1, the air in which was sufficiently replaced by nitrogen, thereafter, in the same manner as in Example 7, heated up to 1,000C. and heat-treated at this temperature to obtain a bundle of carbon-like fiber with a carbon content of 92.8 percent. Thereafter, the said bundle of carbon-like fiber was suspended perpendicularly in a graphite Heat treatment temperature Heattreated tow in air 400 C. 600 C 800 C. 1,000 C.

Heating rate A A B A B A B Treatment with strength increasing agent (0 Measurement:

Heating weight loss (percent)- 16 13 35 42 31 41 48 49 44 47 57 57 57 55 Shrinkage (percent) 8 7 16 17 17 19 19 22 21 21 21 25 25 Denier of monofilament (d.) 3. 8 2. 3 3. 2 1. 9 2. 4 1. 8 2. 4 1. 7 2. 3 1. 8 2. 2 1. 7 2. 0 1. 8 1. 9 1. 6 Drystrength (g./d.) 1. 18 0. 40 1. 90 0. 98 2. 48 l. 68 2. 54 1. 78 2. 62 1. 80 2. 72 1. 80 2. 98 .00 3. l0 2. 20

Dry elongation (percent) 11. 5 10. 7 5. 5 7. 2 2.1 1.8 2.0 1.4 1. 3 1. 5 1. 5 2.0 1. 5 1. 3 1. 7 1. 8

1 Done. 2 None. 0 W

EXAMPLE 7 tube-shaped heating body, a weight was suspended at the Diagonal cloth (thread density: 41 X threads/in thickness 0.77 mm) comprising strong rayon yarn (denier 1.79 d X 750 filament tensile strength 3.55 g/d. ;elongation 17.9 percent) was soaked in an aqueous solution of ammonium sulfite with a concentration of 400 g/l, wrung by a mangle and then dried so as to obtain treated cloth with the adhesion percentage of ammonium sulfite being 37.2 percent. The thus treated cloth was heat-treated in air by passing through the zone at 220C. and the zone at 260C. in a continuous heating furnace equipping infrared heaters so that the total passing time is l.5 hours, then washed and dried. The thus obtained black heat-treated cloth had a tensile strength of 27.5 kg/2.5 cm. of width, and the elementary analysis thereof was as follows.

N=l0.0% S=1.6%

Subsequently, this black heat-treated cloth was charged in the same stainless steel cylinder as in Example 1, the air in which was sufficiently replaced by nitrogen so as to remove the air contained in the said heat-treated cloth, thereafter heated at a heating rate of 2.5C/min. up to 600C. and at 5C/min. over a temperature range of from 600 to 1,000C, and heat-treated at 1,000C. for 1 hour. The tensile strength of heat-treated cloth obtained was 18.5 kg/2.5 cm. of width. Moreover, the strength and elongation of the thread pulled out of this heat-treated cloth and the elementary analysis were as follows:

Tensile strength 2.2 g/d elongation 1.2

elementary analysis C 92.3 H 1.1 O 4.0 N

Subsequently, the thread pulled out of the said heat-treated cloth at 1,000C. (a bundle of filaments) was charged in a graphite tube-shaped heating body (Tamnan furnace), the air in which was then replaced by argon, thereafter heated in order to elevate the temperature from 1,000C. to 2,500C. in 2 hours and heat-treated at 2,500C. for 15 min. As the results, flexible graphite-like filaments with a tensile strength of 1.5 g/d, an elongation of 0.6 percent and a carbon content of 99.9 percent were obtained.

' EXAMPLE8 A bundle of viscose rayon fiber (denier of mono-filament 5.5 d; tensile strength: 1.9 g/d elongation: 21.3 percent) was soaked in an aqueous solution of treating agent comprising 500 g. ofammonium imidosulfonate, 250 g. ofthiourea, 100 g. of aluminum ammonium sulfate and 2,000 cc. of water, thereafter wrung and dried to obtain a bundle of treated fiber with the adhesion percentage of solid component being 48.8 percent. This bundle of treated fiber was heat-treated in air by lower terminal ofsaid bundle as load of 6.0 mg/d and the air in said heating body was sufficiently replaced by argon. Thereafter, the said bundle of carbon-like fiber, in the same way as in Example 7, was heated up to 2,500C. and heattreated. The thus obtained heat-treated fiber was highly flexible graphite-like fiber with a tensile strength of 3.27 g/d, an elongation of 0.48 percent and a carbon content of 99.9 percent.

EXAMPLE 9 The same starting material cloth as in Example 7 was soaked in an aqueous solution of treating agent comprising 490 g. of sulfuric acid, 600 g. of urea, 40 g. of triammonium phosphate and 2,000 cc. of water, wrung by a mangle and then dried to obtain the treated cloth with the adhesion percentage of solid component being 36.6 percent. The thus treated cloth, under the same conditions as in Example 7, was heat-treated in air by passing through a continuous heating furnace equipping infrared ray heaters so as to obtain theblack heattreated cloth in air having a tensile strength of 28.3 kg/2.5 cm. of width. The elementary analysis of this heat-treated cloth was as follows:

Subsequently, this black heat-treated cloth was charged in the same stainless steel cylinder as in Example 7, the air in which was sufiiciently replaced by nitrogen so as to remove the air contained in the said heat-treated cloth, thereafter, under the same conditions as in Example 7, heated up to 1,000C. and heat-treated. Threads were pulled out of the thus obtained heat-treated cloth, and the measurements on the strength and elongation of monofilament and on the elementary analysis thereof were carried out. The results were as follows:

Tensile strength: 2.0 g/d; elongation 1.4

elementary analysis: C 92.0 H 1.2 O 4.1 N=

Subsequently, the thread pulled out of the said heat-treated cloth at 1,000C. (a bundle of filaments) was charged in a graphite tube-shaped heating body, the air in which was replaced by argon, thereafter, in the same way as in Example 7, heated up to 2,500C. and heat-treated so as to obtain highly flexible graphite-like filament with a tensile strength of 1.51 g/d., an elongation of 0.6 percent and a carbon content of 99.9 percent.

What is claimed is:

l. A process for carbonizing cellulose fiber or the products thereof, which comprises the steps of:

a. treating cellulose fiber or the products thereof with a strength increasing agent selected from the group consistheat-treating the product of step (a) at a temperature between about 200C. and about 350C. in an oxidative atmosphere for a period of time sufficient to increase the strength of the fiber; and

. heat-treating the product of step (b) in an inert atmosphere at a temperature of at least about 400C for a period of time sufficient to bring about carbonization.

2. The process of claim 1 in which said strength increasing agent is applied in the form of an aqueous solution.

3. The process of claim 1 in which the organic nitrogen base is selected from the group consisting of urea, a urea derivative, thiourea, a thiourea derivative, and an amine.

4. The process of claim 1 in which the strength increasing agent includes a flame resistance improving agent comprising at least one compound selected from the group consisting of ammonium phosphate, guanidine phosphate, aluminum ammonium sulfate and tetrakis (hydroxymethyl) phosphonium chloride.

5. The process of claim 4 in which the strength increasing agent and flame resistance improving agent are applied in the form of an aqueous solution.

6. The process of claim 1 in which the rate of increase of temperature in said inert atmosphere is between about 1 and about 5 C. per minute. 

2. The process of claim 1 in which said strength increasing agent is applied in the form of an aqueous solution.
 3. The process of claim 1 in which the organic nitrogen base is selected from the group consisting of urea, a urea derivative, thiourea, a thiourea derivative, and an amine.
 4. The process of claim 1 in which the strength increasing agent includes a flame resistance improving agent comprising at least one compound selected from the group consisting of ammonium phosphate, guanidine phosphate, aluminum ammonium sulfate and tetrakis (hydroxymethyl) phosphonium chloride.
 5. The process of claim 4 in which the strength increasing agent and flame resistance improving agent are applied in the form of an aqueous solution.
 6. The process of claim 1 in which the rate of increase of temperature in said inert atmosphere is between about 1* and about 5* C. per minute. 